Carbon dioxide recovery apparatus and method

ABSTRACT

An apparatus for separating and recovering CO 2  from a CO 2  absorbent, includes: a regeneration tower for regenerating the absorbent that has absorbed CO 2  by heating it to separate and remove CO 2  therefrom and to exhaust CO 2  gas; a compressor for compressing the CO 2  gas exhausted from the tower; and a heat exchanger for heating the absorbent in the tower by exchanging heat with a part of the compressed CO 2  by the compressor which is introduced into the tower. The apparatus may include a plurality of the compressors and a plurality of the heat exchangers. The plurality of compressors is arranged in series to sequentially compress the CO 2  gas exhausted from the tower. The plurality of heat exchangers is configured so that each part of the CO 2  compressed by the plurality of compressors is introduced to the tower in parallel to exchange heat with the absorbent in the tower.

BACKGROUND OF THE INVENTION

The present invention relates to an apparatus configured to recovercarbon dioxide (CO₂) and a method for recovering CO₂.

The greenhouse effect due to CO₂ has been regarded as one of the causesof global warming. In order to suppress this phenomenon, much researchand development to prevent or suppress the release of CO₂ into theatmosphere have been carried out. Because CO₂ is generated mainly by thecombustion of fossil fuels, it is desired that exhaust gas generated bythe combustion of fossil fuels be emitted to the atmosphere after CO₂contained in the gas is appropriately reduced or removed therefrom.

Japanese Patent Application Publication No. 2008-62165 discloses anapparatus, which is installed in power generation facilities of athermal power plant, which uses a large amount of fossil fuel, and whichis configured to remove and recover CO₂ contained in exhaust gas bybringing exhaust gas from a boiler into contact with a CO₂ absorbent,and which is configured to store the recovered CO₂ without dischargingthe same into the atmosphere. Japanese Patent Application PublicationNo. 2008-62165 discloses a regeneration tower configured to separate andremove CO₂ gas by applying heat to an absorbent that has absorbed CO₂and is configured to regenerate the absorbent. In addition, JapanesePatent Application Laid-Open No. 2008-62165 discusses a method forcompressing the CO₂ removed in the regeneration tower by using aplurality of compressors to a state in which the compressed CO₂ can beinjected into the ground or into an oilfield to be stored therein.Furthermore, Japanese Patent Application Publication No. 2008-62165discloses a method for improving the energy efficiency of the entire CO₂recovery apparatus by exchanging heat between a part of the absorbentregenerated by the regeneration tower and compression heat generated bya plurality of compressors, by supplying the absorbent heated asdescribed above to a tower bottom portion of the regeneration tower, andby using the heated absorbent supplied to the regeneration tower as aheat source for heating the absorbent.

SUMMARY OF THE INVENTION

The present invention is directed to provide a CO₂ recovery apparatusand a CO₂ recovery method capable of improving energy efficiencycompared with a case of using a conventional method for extracting apart of a regenerated absorbent from a regeneration tower and supplyingthe absorbent heated with compression heat applied by a compressor to atower bottom portion of the regeneration tower.

According to an aspect of the present invention, there is provided anapparatus for separating and recovering CO₂ from a CO₂ absorbent thathas absorbed CO₂, the apparatus including: a regeneration tower forregenerating the CO₂ absorbent that has absorbed CO₂ by heating it toseparate and remove CO₂ therefrom and to exhaust CO₂ gas; a compressorfor compressing the CO₂ gas exhausted from the regeneration tower; and aheat exchanger for heating the CO₂ absorbent in the regeneration towerby executing heat exchange with a part of the compressed CO₂ by thecompressor which is introduced into the regeneration tower.

The apparatus may include a plurality of the compressors and a pluralityof the heat exchangers. The plurality of compressors may be arranged inseries to sequentially compress the CO₂ gas exhausted from theregeneration tower. The plurality of heat exchangers may be configuredso that each part of the CO₂ compressed by the plurality of compressorsis introduced to the regeneration tower in parallel to exchange heatwith the CO₂ absorbent in the regeneration tower.

The compressed CO₂ from one of the plurality of compressors may be usedby one of the plurality of heat exchangers. The used compressed CO₂ fromthe one of the heat exchanger may be compressed by one compressor thatis subsequent to the one compressor.

The heat exchanger may be located in the regeneration tower and may havea shape of a gas pipe through which the compressed CO₂ flows to exchangeheat between the compressed CO₂ and the absorbent flowing in an outerperiphery of the gas pipe. In a case in which a plurality of heatexchangers is located in the regeneration tower, a plurality of gaspipes as the plurality of heat exchangers may be arranged in parallel toone another to exchange heat between the compressed CO₂ flowing thereinand the absorbent flowing in outer peripheries thereof.

The regeneration tower may include a CO₂ desorption unit in which theabsorbent that has absorbed CO₂ falls to remove CO₂ therefrom and awashing unit for cleaning the CO₂ gas removed from the absorbent. Inthis case, the heat exchanger may be located in the CO₂ desorption unitand has a shape of a gas pipe through which the compressed CO₂ flows toexchange heat between the compressed CO₂ and the absorbent flowing in anouter periphery of the gas pipe.

The apparatus may further include a dehydration device for removingmoisture from the compressed CO₂, the dehydration device being locatedbetween two of the plurality of compressors.

According to another aspect of the present invention, there is provideda method for separating and recovering CO₂ from a CO₂ absorbent that hasabsorbed CO₂, comprising the steps of: regenerating the CO₂ absorbentthat has absorbed CO₂ by heating it to separate and remove CO₂therefrom; and compressing CO₂ gas generated by the regeneration of theabsorbent, wherein the step of regenerating the absorbent comprisesheating the CO₂ absorbent by exchanging heat between the CO₂ absorbentand a part of the compressed CO₂ acquired by the step of compressing theCO₂ gas.

The step of compressing the CO₂ gas may include sequentially compressingin two or more stages the CO₂ gas generated by the regeneration of theabsorbent. The step of regenerating the absorbent may include heatingthe CO₂ absorbent by exchanging heat between the CO₂ absorbent and eachof the compressed CO₂ compressed in the two or more stages separatelyfrom one another.

After one of the compressed CO₂ compressed in the two or more stages maybe used for the heat exchange with the CO₂ absorbent and then may beused for the compression in a subsequent stage of the two or morestages.

The method may further include removing moisture from the compressed CO₂between two of the compressions in the two or more stages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrating an embodiment of a CO₂ recoveryapparatus according to the present invention;

FIG. 2 is a cross-sectional view schematically showing an example of aregeneration tower illustrated in FIG. 1; and

FIG. 3 is a cross-sectional view schematically showing another exampleof the regeneration tower illustrated in FIG. 1.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, an embodiment of a CO₂ recovery apparatus and methodaccording the present invention will be described with reference to theattached drawings.

As shown in FIG. 1, a CO₂ recovery apparatus according to the presentembodiment includes, as main components thereof, an absorption tower 10,which is configured to remove CO₂ from gas to be processed by using aCO₂ absorbent, a regeneration tower 20, which is configured toregenerate the CO₂ absorbent by desorbing CO₂ from the CO₂ absorbentwhich has absorbed CO₂ (hereinafter referred to as a “rich absorbent”)(the regenerated CO₂ absorbent will hereafter be referred to as a “leanabsorbent”), and a CO₂ gas compression system 30, which is configured tocompress CO₂ gas generated in the regeneration tower 20. Note thatalthough not illustrated in FIG. 1, the CO₂ recovery apparatus accordingto the present exemplary embodiment can include a desulfurization deviceprovided on the upstream of the absorption tower 10 and which isconfigured to remove sulfur oxides from gas to be processed, inparticular, a high desulfurization cooling tower including a highdesulfurization unit and a desulfurized gas cooling unit.

The absorption tower 10 includes a CO₂ absorption unit 15 in a lowerportion of the absorption tower 10 and a cleaning unit 19 in an upperportion of the absorption tower 10, which are provided across a chimneytray 16 provided in the center portion of the absorption tower 10. Inaddition, the absorption tower 10 includes a gas introduction line 11,which is provided below the CO₂ absorption unit 15 to introduce gas tobe processed into the absorption tower 10, a lean absorbent line 18,which is provided below the CO₂ absorption unit 15 to supply a CO₂absorbent to the CO₂ absorption unit 15, and a rich absorbent line 17,which is provided in the bottom portion of the absorption tower 10 toexhaust the rich absorbent which has absorbed CO₂. Furthermore, theabsorption tower 10 includes a cleaning fluid circulation line 12, whichconnects an upper portion of a cleaning unit 19 with a lower portionthereof to circulatively supply the cleaning fluid accumulated on thechimney tray 16 to the cleaning unit 19, a cooler 13, which is providedon the cleaning fluid circulation line 12 to cool the cleaning fluid,and a gas exhaustion line 14, which is provided on the top portion ofthe absorption tower 10 to exhaust gas that has passed the CO₂absorption unit 15 and the cleaning unit 19 outside the absorption tower10.

The CO₂ absorbent is not limited to a specific type absorbent. However,it is useful to use a CO₂ absorbent containing a basic amine compound asa main component thereof. The basic amine compound includes, forexample, primary amines containing alcoholic hydroxyl such asmonoethanol-amine, and 2-amino-2-methyl-1-propanol; secondary aminescontaining alcoholic hydroxyl such as diethanolamine, 2-methylaminoethanol, and 2-ethylamino ethanol; tertiary amines containingalcoholic hydroxyl such as triethanolamine, N-methyldiethanolamine,2-dimethyl aminoethanol, and 2-diethylaminoethanol; polyethylenepolyamines such as ethylenediamine, triethylenediamine, anddiethylenetriamine; cyclic amines such as piperazines, piperidines, andpyrrolidines; polyamines such as xylylenediamine; and amino acids suchas methylamine carboxylic acid. The CO₂ absorbent can contain one or aplurality of the compounds described above. The concentration of thebasic amine compound can be 10-70% by weight. The CO₂ absorbent cancontain a CO₂ absorption accelerator and a corrosion inhibitor. Inaddition, the CO₂ absorbent can include a medium other than thosedescribed above, such as methanol, polyethylene glycol, or sulfolane.

Tap water or industrial water may be used as the cleaning fluid in thecleaning unit 19, but the present invention is not limited thereto.

The regeneration tower 20 includes a CO₂ desorption unit 21, which isprovided in a portion of the regeneration tower 20 from the center to alower portion thereof. In addition, the regeneration tower 20 includes awashing unit 26, which is provided above the CO₂ desorption unit 21, anda chimney tray 27, which is provided below the CO₂ desorption unit 21.In the regeneration tower 20, the rich absorbent line 17 for introducingthe rich absorbent that has absorbed CO₂ in the absorption tower 10 intothe regeneration tower 20 is provided between the CO₂ desorption unit 21and the washing unit 26. In addition, in the regeneration tower 20, thelean absorbent line 18 for supplying the regeneration-processed leanabsorbent to the absorption tower 10 is provided in the bottom portionof the regeneration tower 20. Furthermore, in the regeneration tower 20,a heat exchanger 28, which exchanges heat between the rich absorbentline 17 and the lean absorbent line 18, is provided. In addition, a heatexchanger 29, which further recovers heat from the lean absorbent, isprovided between the heat exchanger 28 and the absorption tower 10.

The regeneration tower 20 includes an absorbent regeneration line 22 forextracting a part of the lean absorbent from the bottom portion of theregeneration tower 20 and for supplying the extracted lean absorbent tothe chimney tray 27 from above. The absorbent regeneration line 22includes a reboiler 23, which heats the lean absorbent. In addition, theregeneration tower 20 includes a CO₂ gas exhaustion line 24 forexhausting CO₂ gas which has been desorbed from the lean absorbent fromthe top portion of the regeneration tower 20. The CO₂ gas exhaustionline 24 includes a condenser 25 which condenses steam entrained in theCO₂ gas and a separator drum 31A which separates condensed water, whichresults from the condensation by the condenser 25, from the gas. Thecondenser 25 can use cooling water to cool the gas. A condensed waterreturn line 39, which is a line for supplying the separated condensedwater as washing water for the washing unit 26 of the regeneration tower20, is provided to the separator drum 31A. A pump 46 for feeding thecondensed water to the regeneration tower 20 is provided to thecondensed water return line 39. Note that an inner configuration of theregeneration tower 20, particularly the CO₂ desorption unit 21, will bedescribed in detail later below.

The CO₂ gas compression system 30 includes a plurality of compressors 33as a main component thereof. The plurality of compressors 33 is seriallyprovided and is configured to compress the CO₂ gas exhausted from theregeneration tower 20. In FIG. 1, four compressors 33A through 33D areillustrated. However, in the present exemplary embodiment, the number ofthe compressors 33 is not limited to four. More specifically, thecompressors 33 can be provided in the appropriate number with which CO₂gas can be serially compressed in raising the pressure applied to thecompressed CO₂ gas to a predetermined pressure level (for example, acritical point of 7.4 MPa). For example, if compressed CO₂ gas is to beinjected from the ground of an oilfield and reserved therein, it isuseful if the number of the compressors 33 to be provided is three. Itis more useful, in this case, if the number of the compressors 33 to beprovided is four or more. More specifically, it is useful if the numberof the compressors 33 is eight or less.

To each of the compressors 33, a separator drum 31 which separates thecondensed water from the CO₂ gas, a gas line 32 for feeding the CO₂ gasfrom which condensed water has been separated to the compressor 33, acompressed CO₂ gas-heat utilizing line 34 for feeding a part of thecompressed CO₂ gas compressed by the compressor 33, a compressed CO₂ gascompression line 36 for supplying a part of the CO₂ gas compressed bythe compressor 33 to a subsequent compressor, and a compressor 37 whichcondenses steam entrained in the compressed CO₂ gas are provided. Inaddition, a compressed CO₂ gas recovery line 38 for returning thecompressed CO₂ gas fed to the regeneration tower 20 to the compressedCO₂ gas compression line 36 is provided before the condenser 37 of thecompressed CO₂ gas compression line 36. Although not illustrated in FIG.1, a condensed water return line for returning the condensed water tothe regeneration tower 20 is provided to each of the second-stagethrough the fourth-stage separator drums 31 (i.e., separator drums 31Bthrough 31D) as is provided to the separator drum 31A.

The CO₂ gas compression system 30 further includes a dehydration device41, which is capable of removing moisture from the compressed CO₂ gas byan amount larger than a saturating amount. The dehydration device 41 isnot limited to a particular type device and any dehydration devicecapable of reducing the humidity of gas can be used. For example, it isuseful if a dehydration device of a type that uses a glycol absorbent isused. For the absorbent described above, monoethylene glycol, diethyleneglycol, or triethylene glycol, for example, can be used. A separatordrum 42 can be provided upstream of the dehydration device 41. Thedehydration device 41 can perform at a sufficiently high level bypreviously removing the moisture from the gas as large an amount as thesaturating amount by using the separator drum 42. Note that it is usefulif the dehydration device 41 is provided and positioned in the center ofthe location where the plurality of compressors 33 is provided. Forexample, in the example illustrated in FIG. 1, the dehydration device 41is provided between the second-stage compressor 33B and the third-stagecompressor 33C. However, the present invention is not limited to this.

Referring to FIG. 2, to the CO₂ desorption unit 21 of the regenerationtower 20, a plurality of gas pipes 51, through which the compressed CO₂gas compressed by the plurality of compressors 33 of the CO₂ gascompression system 30 is fed, is provided in a manner extending from atower bottom 20B towards a tower top 20A in parallel to one another. TheCO₂ desorption unit 21 provided in the outer periphery of the gas pipe51 is a portion onto which the rich absorbent diffused from the richabsorbent line 17 via a plurality of nozzles falls. More specifically,the CO₂ desorption unit 21 is configured to heat, via the gas pipe 51,the rich absorbent outside the gas pipe 51 by using the compressed CO₂gas flowing through the gas pipe 51.

Gas pipes 51A through 51D are connected to compressed CO₂ gas heatutilizing lines 34A through 34D, which are extended from compressors 33Athrough 33D, respectively. With this configuration, portions of thecompressed CO₂ gas, which have been compressed by different compressors,may not be mixed together or introduced into one gas pipe. In addition,the compressed CO₂ gas heat utilizing lines 34A through 34D areconnected to compressed CO₂ gas recovery lines 38A through 38Drespectively via the gas pipes 51A through 51D. Note that in the exampleillustrated in FIG. 2, four gas pipes 51A through 51D are provided tothe CO₂ desorption unit 21. However, this is a simplified illustrationpresented merely for easier understanding. More specifically, aplurality of gas pipes 51 can be provided for each compressed CO₂ gas tobe compressed by each compressor. It is useful if the total number ofthe gas pipes 51 is in the range of several tens to several hundreds.

The outer periphery portion of the gas pipe 51 of the CO₂ desorptionunit 21 can be filled with a filling (not illustrated). With thisconfiguration, the desorption of the CO₂ gas from the rich absorbent canbe accelerated. In addition, a fin (not illustrated) can be provided onan outer wall of the gas pipe 51. With this configuration, the heatexchange between the gas flowing inside the pipe and the absorbentflowing outside the pipe can be accelerated.

With the above-described configuration, as illustrated in FIG. 1, at thestart of processing, gas to be processed containing CO₂ is introduced tothe absorption tower 10 via the gas introduction line 11. In addition,CO₂ absorbent is supplied to the absorption tower 10 via the leanabsorbent line 18. In the CO₂ absorption unit 15, the gas to beprocessed and the CO₂ absorbent are brought into gas-liquid contact witheach other to allow the CO₂ absorbent to absorb CO₂ contained in the gasto be processed to remove CO₂ therefrom. The gas, from which CO₂ hasbeen removed, flows above the chimney tray 16 to reach the cleaning unit19, which is provided in the upper portion of the absorption tower 10.The gas is then cleaned by using the cleaning fluid. Subsequently, thecleaned gas is exhausted from the gas exhaustion line 14, which isprovided in the top portion of the absorption tower 10. The cleaningfluid used by the cleaning unit 19 is accumulated on the chimney tray 16and does not fall into the CO₂ absorption unit 15. The cleaning fluidaccumulated on the chimney tray 16 is fed to the cooler 13 via thecleaning fluid circulation line 12 to be cooled by the cooler 13. Afterthat, the cooled cleaning fluid is reused by the cleaning unit 19 as thecleaning fluid.

The rich absorbent, which has absorbed CO₂ in the absorption tower 10,is exhausted from the bottom portion of the absorption tower 10 via therich absorbent line 17. Subsequently, the exhausted rich absorbent isheated by the heat exchanger 28 to be fed to the regeneration tower 20.As illustrated in FIG. 2, in the regeneration tower 20, the richabsorbent is diffused onto the CO₂ desorption unit 21 from the pluralityof nozzles provided on a tip of the rich absorbent line 17. The richabsorbent falls while being heated by the plurality of gas pipes 51provided to the CO₂ desorption unit 21. Because a high-temperaturecompressed CO₂ gas flows from the tower bottom 20B towards the tower top20A inside the gas pipe 51, the rich absorbent is exposed to a highertemperature as the rich absorbent falls more downwards along the CO₂desorption unit 21. Subsequently, the rich absorbent falls to reach thechimney tray 27, which is provided in the vicinity of the tower bottom20B after discharging most of the CO₂.

The absorbent accumulated onto the chimney tray 27 is fed from an inlet22B of the absorbent regeneration line 22 to be heated by the reboiler23 with steam. Subsequently, residual CO₂ is discharged from theabsorbent to regenerate the absorbent. Then the regenerated absorbent isreturned to the tower bottom 20B of the regeneration tower 20 from anoutlet 22A of the absorbent regeneration line 22. The regenerated leanabsorbent is fed to the heat exchanger 28 from the tower bottom 20B viathe lean absorbent line 18. The heat exchanger 28 heats the richabsorbent. Subsequently, the heat exchanger 29 further recovers the heatfrom the absorbent. Then the absorbent is supplied to the absorptiontower 10.

The CO₂ gas desorbed from the rich absorbent is fed through the chimneytray 27 and the CO₂ desorption unit 21 and is raised up to the washingunit 26. In the washing unit 26, the washing water is diffused from aplurality of nozzles provided on a tip of the condensed water returnline 39 to remove the entrained absorbent from the CO₂ gas. After beingwashed by the washing unit 26, CO₂ gas is exhausted from the CO₂ gasexhaustion line 24, which is provided on a tower top 20A of theregeneration tower 20.

In the CO₂ gas exhaustion line 24, steam entrained in the CO₂ gas isinitially condensed by the condenser 25. Furthermore, the condensedwater is separated by the separator drum 31A. After being separated, thecondensed water is returned to the regeneration tower 20 via thecondensed water return line 39. After the condensed water is removedfrom the CO₂ gas, the CO₂ gas is introduced into the first-stagecompressor 33A via a CO₂ gas line 32A. In the compressor 33A, thetemperature of the CO₂ gas is raised to a high level when the CO₂ gas iscompressed to a predetermined pressure. After the CO₂ gas is compressedby the compressor 33A, a part of the compressed CO₂ gas is fed to theregeneration tower 20 via the compressed CO₂ gas heat utilizing line 34Ato utilize the same as a heat source for heating the absorbent. On theother hand, the other portion of the compressed CO₂ gas is fed to thesecond-stage compressor 33B via the compressed CO₂ gas compression line36A to be further compressed. In addition, the compressed CO₂ gas thathas heated the absorbent in the regeneration tower 20 is recovered viathe compressed CO₂ gas recovery line 38A. Then, the recovered compressedCO₂ gas is mixed with compressed CO₂ gas upstream of the condenser 37Aof the compressed CO₂ gas compression line 36A.

In the compressed CO₂ gas compression line 36A, similar to theprocessing by the first-stage compressor 33A, the steam entrained in thecompressed CO₂ gas is condensed by a condenser 37A. Then, the condensedwater is separated by the separator drum 31B. Subsequently, the CO₂ gasis introduced into the second-stage compressor 33B via a CO₂ gas line32B. The second-stage compressor 33B compresses the CO₂ gas to apredetermined pressure and the temperature of the gas is raised.Subsequently, a part of the further-compressed CO₂ gas is fed to theregeneration tower 20 via the compressed CO₂ gas utilizing line 34B tobe utilized as a heat source for heating the absorbent. The other partof the compressed CO₂ gas and the compressed CO₂ gas that has heated theabsorbent in the regeneration tower 20 are fed to the third-stagecompressor 33C via the compressed CO₂ gas compression line 36B and thecompressed CO₂ gas recovery line 38B to be further compressed thereby.

Note that before the compressed CO₂ gas is compressed by the third-stagecompressor 33C, entraining steam is condensed and removed by a condenser37B and the separator (dehydration) drum 42 before the compressed CO₂gas is introduced to the dehydration device 41. In the dehydrationdevice 41, the moisture in the compressed CO₂ gas can be absorbed by theabsorbent to be removed therefrom by bringing the compressed CO₂ gasinto gas-liquid contact with a glycol absorbent. After the compressedCO₂ gas is dehydrated by the dehydration device 41, the entrainingmoisture and glycol are condensed and separated by the separator drum31C. Subsequently, the compressed CO₂ gas is introduced into thehigh-pressure side compressor 33C. In the third-stage compressor 33C,the compressed CO₂ gas is compressed with a pressure as high as apredetermined pressure and the temperature of the gas is raised.Subsequently, a part of the compressed CO₂ gas is fed to theregeneration tower 20 via the compressed CO₂ gas utilizing line 34C toutilize the fed part of the compressed CO₂ gas as the heat source forheating the absorbent. In addition, the other portion of the compressedCO₂ gas and the compressed CO₂ gas used by the regeneration tower 20 asthe heat source are fed to the fourth-stage compressor 33D via thecompressed CO₂ gas compression line 36C and the compressed CO₂ gasrecovery line 38C. In the fourth-stage compressor 33D, in the mannersimilar to the processing described above, after moisture and glycol arecondensed and removed from the compressed CO₂ gas, the compressed CO₂gas is further compressed with a pressure as high as a finalpredetermined pressure and the compressed CO₂ is utilized as the heatsource for heating the absorbent in the regeneration tower 20. Thecompressed CO₂ compressed with the final predetermined pressure isrecovered to be used for a predetermined purpose.

By compressing the CO₂ gas exhausted from the regeneration tower 20 by aplurality of stages by using the plurality of compressors 33 until thepressure reaches the final predetermined pressure and by utilizing thecompressed CO₂ gas compressed by each compressor 33 or a part of thecompressed CO₂ as the heat source for heating the absorbent in theregeneration tower 20, a part of the energy consumed in compressing theCO₂ gas by using the compressor 33 can be used in heating the absorbentin the regeneration tower 20. Therefore, the amount of steam necessaryfor heating and regenerating the absorbent in the regeneration tower 20can be reduced. Accordingly, the total amount of the energy consumed bythe entire CO₂ recovery apparatus can be reduced.

Furthermore, because the temperature of the compressed CO₂ gas utilizedas the heat source for heating the absorbent in the regeneration tower20 or the compressed CO₂ is decreased, the amount of cooling waternecessary for the cooling by the condenser 37 can be reduced by mixing,in a portion upstream of the condenser 37, the utilized compressed CO₂gas or the utilized compressed CO₂ with compressed CO₂ gas or compressedCO₂ yet to be utilized as the heat source. In addition, in some cases inwhich the cooling by the condenser 37 is unnecessary, the cooling by thecondenser 37 can be omitted.

Now, shutdown of the CO₂ recovery apparatus according to the presentexemplary embodiment will be described. In shutting down the absorptiontower 10 and the regeneration tower 20, at the start of the shutdownoperation, the CO₂ gas compression system 30 is shut down and thereboiler 23 of the regeneration tower 20 is shut down. In the shutdownoperation described above, a switching valve (not illustrated) isswitched so that the CO₂ gas from the CO₂ gas exhaustion line 24provided on the tower top 20A of the regeneration tower 20 is exhaustedoutside the system. At the same time, the compressed air within theplant is supplied to the CO₂ gas line 32A.

By executing the operation described above, non-compressed CO₂ gas isfed through the gas pipe 51 of the regeneration tower 20. Because thecompressed air acts as a coolant due to its low temperature ranging from20 to 40° C., the inside of the regeneration tower 20, whose temperaturehas been raised to a high level by 23, can be cooled more rapidly thanthe time taken in the case of a conventional method. Accordingly, thetime taken in shutting down the CO₂ recovery apparatus can be shortened.At the same time, because the inside of the compressor 33 can besubstituted with the dry compressed air, the moisture within thecompressor 33 can be removed. As a result, corrosion of the compressor33 can be prevented.

The heating of the CO₂ desorption unit 21 of the regeneration tower 20by using the compressed CO₂ gas or the compressed CO₂ compressed by thecompressor 33 is as described above with reference to FIG. 2. However,as illustrated in FIG. 3, the entire regeneration tower 20 including theCO₂ desorption unit 21 and the washing unit 26 can be heated with thecompressed CO₂ gas or the compressed CO₂. To paraphrase this, the entireregeneration tower 20 can be heated as described above because theplurality of the gas pipes 51 is provided in a manner extending from thetower bottom portion, in which the CO₂ desorption unit 21 is provided,towards the tower top portion, in which the washing unit 26 is provided,in parallel to one another.

Although not illustrated in the drawings, a high desulfurization coolingtower configured to remove sulfur oxides contained in gas to beprocessed, which can be provided upstream of the gas to be processed bythe absorption tower 10, will now be described below. The highdesulfurization cooling tower includes a high desulfurization unit,which is provided in a lower portion of the high desulfurization coolingtower and configured to remove to a high degree a sulfur oxidescontained in gas to be processed. In addition, the high desulfurizationcooling tower includes a desulfurized gas cooling unit, which isprovided in an upper portion of the high desulfurization cooling towerand configured to cool the desulfurized gas that has passed the highdesulfurization unit down to 50° C., for example. The gas that haspassed the desulfurized gas cooling unit is then introduced into theabsorption tower 10. Exhaust gas usually contains sulfur oxides andcarbon dioxide. The high desulfurization unit executes highdesulfurization processing to reduce the concentration of the sulfuroxides down to 5 ppm or lower (it is more useful if the concentration ofthe sulfur oxides is reduced to 1 ppm or lower) by absorbing andremoving sulfur oxides contained in exhaust gas by bringing the exhaustgas into contact with a basic absorbent. If the concentration of sulfuroxides contained in the gas exceeds 5 ppm, the sulfur oxides may beaccumulated into the CO₂ absorbent used in the absorption tower. In thiscase, a problem of the increased frequency of reclaiming the CO₂absorbent may arise. To prevent the above-described problem, the highdesulfurization unit executes the high desulfurization processing.

For the basic absorbent, an absorbent that contains one basic compoundor a mixture of two or more compounds of calcium carbonate, calciumhydroxide, magnesium hydroxide, sodium hydroxide, and the like can beused, for example. It is useful, for example, if the concentration ofthe basic compound is 0.1 to 30% by weight.

The desulfurized gas cooling unit cools the gas processed by the highdesulfurization unit down to 50° C. or lower, more usefully to 45° C. orlower, and yet more usefully to a range of 30-45° C. The desulfurizedgas cooling unit cools the highly desulfurization-processed gas becauseotherwise a problem of wasteful consumption of the basic amine compoundmay arise due to an increase, in the absorption tower 10 which is atower subsequent to the high desulfurization cooling tower, of theamount of the basic amine compound, which is the main component of theCO₂ absorbent entrained in the gas. As described above, by installingthe high desulfurization cooling tower upstream of the gas to beprocessed by the absorption tower 10 of the present invention, theabsorption tower 10 can easily remove and recover CO₂ contained in thegas at low costs.

Example

The inventor of the present invention carried out a simulation for aratio of reduction of the amount of steam consumed by the reboilerbetween a case in which the absorbent is heated with the compressed CO₂gas or the compressed CO₂ by using the CO₂ desorption unit of theregeneration tower illustrated in FIG. 2 and a case in which theabsorbent is heated with the compressed CO₂ gas or the compressed CO₂ byusing the entire regeneration tower illustrated in FIG. 3, compared witha comparative example that does not execute the heating with thecompressed CO₂ gas or the compressed CO₂. For conditions of thesimulation, the number of the stages of the compressor was set at four.In addition, at the respective stages, the levels of compressing the CO₂were set at 0.5 MPa, 1.9 MPa, 5.5 MPa, and 10 MPa. The ratios of thecompressed CO₂ gas or the compressed CO₂ supplied to the regenerationtower by the respective compressors were uniformly set at 100% on theflow rate basis.

As a result of the simulation, the inventor found that in executing theheat exchange by using the CO₂ desorption unit illustrated in FIG. 2only, the amount of the steam consumed by the reboiler was reduced by aratio as high as 8.1% in comparison with the comparative example. Inaddition, the inventor found that in executing the heat exchange byusing the entire regeneration tower illustrated in FIG. 3, the amount ofthe steam consumed by the reboiler was reduced by a ratio as high as7.5% in comparison with the comparative example.

Whereas the present invention has been described in connection withpreferred embodiments, it is not intended to limit the scope of thepresent invention to a specific embodiment described above. In addition,it is intended that various modifications, alterations, or equivalentscan implement the present invention without any deviation from thespirit and the scope of the present invention as defined by the appendedclaims.

What is claimed is:
 1. An apparatus for separating and recovering CO₂ from a CO₂ absorbent that has absorbed CO₂ comprising: a regeneration tower for regenerating the CO₂ absorbent that has absorbed CO₂ by heating it to separate and remove CO₂ therefrom and to exhaust CO₂ gas; a compressor for compressing the CO₂ gas exhausted from the regeneration tower; and a heat exchanger for heating the CO₂ absorbent in the regeneration tower by executing heat exchange with a part of the compressed CO₂ by the compressor which is introduced into the regeneration tower.
 2. The apparatus according to claim 1, comprising a plurality of the compressors and a plurality of the heat exchangers, wherein the plurality of compressors is arranged in series to sequentially compress the CO₂ gas exhausted from the regeneration tower, and wherein the plurality of heat exchangers is configured so that each part of the CO₂ compressed by the plurality of compressors is introduced to the regeneration tower in parallel to exchange heat with the CO₂ absorbent in the regeneration tower.
 3. The apparatus according to claim 2, wherein the compressed CO₂ from one of the plurality of compressors is used by one of the plurality of heat exchangers, and wherein the used compressed CO₂ from the one of the heat exchanger is compressed by one compressor that is subsequent to the one compressor.
 4. The apparatus according to claim 1, wherein the heat exchanger is located in the regeneration tower and has a shape of a gas pipe through which the compressed CO₂ flows to exchange heat between the compressed CO₂ and the absorbent flowing in an outer periphery of the gas pipe.
 5. The apparatus according to claim 2, wherein the plurality of heat exchangers is located in the regeneration tower and each has a shape of a gas pipe through which the compressed CO₂ flows to exchange heat between the compressed CO₂ and the absorbent flowing in outer peripheries of the gas pipes, and wherein the plurality of gas pipes is arranged in parallel to one another.
 6. The apparatus according to claim 1, wherein the regeneration tower comprises a CO₂ desorption unit in which the absorbent that has absorbed CO₂ falls to remove CO₂ therefrom and a washing unit for cleaning the CO₂ gas removed from the absorbent.
 7. The apparatus according to claim 6, wherein the heat exchanger is located in the CO₂ desorption unit and has a shape of a gas pipe through which the compressed CO₂ flows to exchange heat between the compressed CO₂ and the absorbent flowing in an outer periphery of the gas pipe.
 8. The apparatus according to claim 1, further comprising a dehydration device for removing moisture from the compressed CO₂, the dehydration device being located between two of the plurality of compressors.
 9. The apparatus according to claim 8, wherein the dehydration device removes moisture from the compressed CO₂ by using a glycol absorbent.
 10. A method for separating and recovering CO₂ from a CO₂ absorbent that has absorbed CO₂, comprising the steps of: regenerating the CO₂ absorbent that has absorbed CO₂ by heating it to separate and remove CO₂ therefrom; and compressing CO₂ gas generated by the regeneration of the absorbent, wherein the step of regenerating the absorbent comprises heating the CO₂ absorbent by exchanging heat between the CO₂ absorbent and a part of the compressed CO₂ acquired by the step of compressing the CO₂ gas.
 11. The method according to claim 10, wherein the step of compressing the CO₂ gas comprises sequentially compressing in two or more stages the CO₂ gas generated by the regeneration of the absorbent, and wherein the step of regenerating the absorbent comprises heating the CO₂ absorbent by exchanging heat between the CO₂ absorbent and each of the compressed CO₂ compressed in the two or more stages separately from one another.
 12. The method according to claim 11, wherein after one of the compressed CO₂ compressed in the two or more stages is used for the heat exchange with the CO₂ absorbent and then is used for the compression in a subsequent stage of the two or more stages.
 13. The method according to claim 10, further comprising removing moisture from the compressed CO₂ between two of the compressions in the two or more stages.
 14. The method according to claim 13, wherein the step of removing moisture comprises dehydrating the compressed CO₂ by using a glycol absorbent. 